1. Field of the Invention
The present invention generally relates to field emitter array devices and more particularly to a field emitter array device configured by a plurality of cathodes arranged in the form of a matrix.
2. Description of Related Art
A field emitter array causes an emission of electrons by inducing a deformation in the surface potential of a cathode. There, an intensive electric field is applied on the cathode, and electrons in the cathode are emitted therefrom by passing through the deformed potential barrier by the tunneling effect. To accomplish the emission of electrons, the field emitter array includes an electron beam source that in turn includes a cathode to which a negative voltage is applied and a gate electrode provided adjacent to the cathode for inducing an intensive electric field thereto. After emission from the cathode, the electrons are accelerated and captured by an anode electrode. The electron beam source of such a configuration can be fabricated with sizes on the order of several microns by using the microfabrication technique employed commonly in the fabrication of semiconductor devices. Thereby, it is possible to arrange minute electron-beam sources in a matrix shape over an extensive area. The field emitter array of such a configuration is expected to be used in high-speed arithmetic devices or high-speed and high-luminosity flat display devices.
FIG. 1 is a perspective view schematically illustrating a conventional field emitter array.
Referring to FIG. 1, a field emitter array is formed on an insulating base 10, and an insulating layer 11 is formed on the upper major surface of the base 10. There, a plurality of cathode electrodes 12 are formed on the lower major surface of the insulating layer 11 to extend in a first direction with a parallel relationship to each other. Further, a plurality of gate electrodes 13 are formed on the upper major surface of the above-mentioned insulating layer 11 to extend in a direction approximately perpendicular to the first direction, with a parallel relationship to each other. Electron beam generating sources 14 are formed in the above-mentioned insulating layer 11 in correspondence to the positions where the above-mentioned cathode electrodes 12 and the gate electrodes 13 intersect with each other. In an example shown in the FIG. 1, each of the electron beam sources 14 is formed of a plurality of electron-beam source elements. The entire apparatus shown in FIG. 1 is housed in a sealed vacuum vessel not illustrated.
FIG. 2 is an enlarged view of one of the electron-beam sources of FIG. 1.
Referring to FIG. 2, an electron-beam source 14 is provided in the insulating layer 11 typically made of silicon oxide in correspondence to a through-hole 11a formed at a position in correspondence to an intersection of the above-mentioned cathode electrode 12 and the gate electrode 13. The beam source 14 includes an emitter tip having a pointed cone shape. Typically, the emitter tip 15 is formed of Mo, and is formed on the cathode electrode 12. As shown in FIG. 2, the gate electrode 13 extends from the side wall of the through-hole 11a toward the emitter tip 15, and forms a narrow gap between itself and the emitter tip 15. By applying a positive voltage on the gate electrode 13 and simultaneously a negative voltage on the cathode electrode 12, an intensive electric field is established between the gate electrode 13 and the emitter tip 15. Such an electric field induces a deformation in the potential barrier on the surface of the emitter tip 15 and allows electrons in the emitter tip 15 to be emitted by the tunneling effect. Electrons thus emitted are accelerated by a positive voltage applied to an anode (not shown in FIGS. 1 and 2) provided opposite to the base 10, and are subsequently captured by the anode. When a fluorescent coating is provided in the vicinity of the anode, a visible image is formed according to a pattern of the emitted electron beam and the device can be used as a flat display panel. Such a flat display panel can be formed for example by forming the anode by a transparent conductive body coated with a fluorescent substance.
In such a field emitter array, it will be easily understood that a degradation in the electron beam emission occurs when a volatile substance such as a gas is absorbed by the emitter tip 15. Therefore, it is desirable and essential in the field emitter array to effect a cleaning process of the emitter tip 15 at predetermined intervals or at every start-up of the apparatus. In the vacuum tubes, it is generally practiced to provide a getter in the vacuum container for absorbing gas. On the other hand, in the field emitter array that does not use the thermal emission of electrons, the mere provision of a getter in the container is not sufficient to ensure satisfactory cleaning. Further, it should be noted that the external heating of the field emitter array shown in FIG. 1 is generally impossible once the field emitter array is assembled in an electronic apparatus.
FIG. 3 illustrates a process for cleaning the emitter tip 15 in a field emitter array which process is described in the Japanese Laid-open Patent Application No. 4-22038. It should be noted that the laid-open publication of the foregoing patent reference has occurred after the basic application of the present application has been filed. In FIG. 3, the base 10 is omitted for the sake of convenience of illustration. In this conventional method, an excitation voltage is applied across a pair of neighboring electron-beam sources 14a and 14b so that an electron beam is formed originating from the electron-beam source 14a and reaching the electron-beam source 14b. As a result, a volatile contaminant absorbed in the emitter tip 15b in the electron-beam source 14b is evaporated due to the energy of the electron-beam and is absorbed by a getter provided in the container.
Referring to FIG. 3, a negative voltage is applied to a cathode electrode 12a of the electron-beam source 14a, and a positive voltage is applied to a cathode electrode 12b of the neighboring electron-beam source 14b. An intense voltage is thereby applied between an emitter tip 15a formed on the cathode electrode 12a and an emitter tip 15b formed on the cathode electrode 12b. When that voltage reaches a level high enough to excite field emission of electrons in the emitter tip 15a, an electron beam is formed from the emitter tip 15a to the emitter tip 15b, and the energy of the beam causes a volatile substance on the emitter tip 15b to evaporate.
While the above-mentioned prior art reference does not make any reference to a voltage applied to the anode while effecting a cleaning process, it is a general practice to apply a positive voltage to the anode. FIG. 4 illustrates a potential distribution when applying a positive voltage to the anode of the electron-beam source shown in FIG. 3, wherein it should be noted that FIG. 4 is reversed left to right in relation to FIG. 3. It is assumed in the computations in FIG. 4 that the gate electrodes 13a and 13b are both grounded.
As can be seen from FIG. 4, under the condition that a positive voltage is applied to the anode, electrons emitted from the emitter tip 15b are mainly attracted by the anode electrode, even when a positive voltage is applied to the emitter tip 15a, and hardly ever reach the emitter tip 15a. In other words, a voltage applied to the anode electrode, provided opposite to the electron-beam source, exercises an essential influence on the efficiency of the cleaning process.